This inventioon relates to heat exchangers utilizing air flow suitable for use with air conditioners, refrigerating apparatus, dehumidifiers and the like, and more particularly it is concerned with a cross finned tube exchanger including a multiplicity of substantially parallel, closely spaced fin plates formed with stepped louvers thereon.
A cross finned tube heat exchanger essentially comprises a multiplicity of fin plates of a suitable area formed of thin sheet metal stock, such as aluminum, and having a plurality of aligned openings formed therein, and a plurality of heat transfer tubes inserted in the aligned openings in the fin plates substantially parallel and closely spaced apart from one another with a pitch of several millimeters. After insertion in the aligned openings, the heat transfer tubes are expanded into intimate contact with the fin plates and connected at their ends by U-shaped bent tube portions to form a bundle of heat transfer tubes of a suitable number arranged in serpentine form to provide a passage for a heat transfer medium subjected to heat exchange with another heat transfer medium flowing across the surfaces of the fin plates. More specifically, cold water, hot water or a refrigerant flows through the heat transfer tubes while air flows across the surfaces of the fin plates substantially parallel to one another, so that the two fluids exchange heat with one another through the walls of the tubes and the fin plates.
The cross finned tube heat exchanger of the aforesaid construction offers the advantages that it has a relatively large heat transfer area and a relatively compact size. However, in this type of heat exchanger, some problems are raised which will be described hereinafter.
An overall heat transfer coefficient of a heat exchanger is determined by heat transfer between a heat transfer medium flowing through the heat transfer tubes and the walls of the heat transfer tubes and heat transfer between currents of external air and the surfaces of the fins. However, the latter heat transfer being lower in degree than the former heat transfer, the overall heat transfer coefficient is largely determined by the heat transfer action of the currents of external air flowing across the surfaces of the fins.
Boundary layers of air are formed on the surfaces of fins arranged substantially parallel to one another and in the currents of external air flowing across the surfaces, and such boundary layers cause a reduction in heat transfer characteristics as their thicknesses become large. The thickness of each boundary layer of air currents or temperature boundary layer becomes greater in going from the upstream end of each fin toward the downstream end of the fin until the temperature boundary layers formed on the surfaces of the two adjacent fins merge into one in a position spaced apart a certain distance from the upstream end of each fin toward the downstream end of the fin. Thus the heat transfer occurring between the surfaces of the fin plates and external air is reduced in degree in going from the upstream end toward the downstream end of the fin plates, so that the surface of each fin has a low heat transfer coefficient on an average.
As described hereinabove, a cross finned tube heat exchanger using flat fin plates has a low heat transfer coefficient because of the presence of boundary layers formed on the surfaces of the fin plates. It is effective to prevent formation of the boundary layers to increase the heat transfer coefficient of the surface of each fin plate through which heat transfer occurs between the two heat transfer mediums.
Various proposals have been made to improve the fin plates in view of the foregoing. In U.S. Pat. Nos. 3,380,518, 3,397,741 and 3,438,433, a multiplicity of flat-plate louvers are formed on each of a multiplicity of flat fin plates and arranged in a direction at right angles to the currents of air flow across the surfaces of the fin plates. In these heat exchangers, when air flows across the surfaces of the fin plates, a temperature boundary layer is formed on the surface of each fin plate. The temperature boundary layer ceases its growth at the rear end of each louver and its thickness is greatly reduced by the effect of the principal air current until the temperature boundary layer reaches the next following louver. Because of the development of this air flow pattern on the surface of the fin plate, the leading edge effect of the louver is increased so that the air current is cut by the front edge of the louver and the fins with louvers have a relatively high heat exchange coefficient as compared with the flat fins. It is to be noted that the louvers themselves are flat in shape in these heat exchangers.
Japanese Utility Model Application Laid-open No. 17867/78 discloses a heat exchanger comprising fins bent into convoluted form in the flow direction of external air so that ridges and valleys are formed to extend in a direction at right angles to the external air flow, and a multiplicity of louvers arranged in the same direction as the ridges on the fins in such a manner that they are elevated from the surfaces of the fins and parallel thereto.
In this heat exchanger, the turbulent flow promoting function of the convoluted fins and the leading edge effect of the louvers to cut the air currents are combined to increase the heat exchange efficiency of the fins. It is to be noted, however, that the louvers themselves are flat in shape.
U.S. Pat. No. 3,796,258 discloses a heat exchanger including convoluted fin plates formed with a plurality of air passage openings. This heat exchanger also increases the heat exchange efficiency by destroying the boundary layers of air currents formed on the surfaces of the fin plates and by forming turbulence in the air currents flowing across the surfaces of the fin plates. This heat exchanger has no louvers.
The prior art referred to hereinabove provides an increase in the heat exchange efficiency. However, a further increase in heat exchange efficiency is desired in cross finned tube heat exchangers. The provision of louvers reduces the strength of the fin plates, particularly the rigidity of fin plates longitudinally of the louvers because of a multiplicity of cuts made therein. In view of the recent tendency to reduce the thickness of the fin plates and with a view to facilitating assembling of heat exchangers, the reduction in the rigidity of the fin plates constitutes a serious defect. It is also earnestly desired that the fin plates be increased in strength.